Nitrate in a Farm Well
A rural family draws drinking water from a shallow well beside a 40-hectare canola field. Testing shows nitrate-nitrogen at 14 mg/L; the guideline for nitrate-nitrogen in drinking water is 10 mg/L. The well casing is 6 m deep and the water table sits 4 m below grade. The family includes a pregnant adult and a 2-year-old. The field is fertilized each spring with ammonium nitrate at 120 kg N/ha, and the nearest edge of the field is 15 m from the well. A neighbour's well 300 m away tests at 3 mg/L. The family asks whether they can keep drinking the water while they decide what to do.
1. Identify the most probable source of the nitrate and explain the evidence that supports your conclusion.
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The most probable source is the fertilized field, delivered by shallow groundwater flow. Three lines of evidence converge:
- Concentration gradient. The family well (14 mg/L) is far above the neighbour's well (3 mg/L) only 300 m away. A regional or geological source would not produce such a sharp local contrast; a nearby surface source does.
- Depth and vulnerability. The water table is only 4 m down and the casing only 6 m, so the well screens shallow, recently recharged water with little soil or clay to denitrify it. Nitrate is mobile and conservative in oxidized groundwater, so it travels with recharge rather than being filtered out.
- Land use and timing. Ammonium nitrate at 120 kg N/ha on a 15 m setback is a large nitrogen load close to the well. Spring application followed by snowmelt or rain recharge matches the timing of contamination.
Alternative sources (septic systems, animal waste, atmospheric deposition) should be ruled out by a land-use survey, but the spatial pattern and shallow screened interval make agricultural nitrogen the leading hypothesis. Confirming with a nitrogen isotope ratio or a second sampling round would strengthen the attribution.
2. Recommend immediate actions for this household and justify each with reference to the measured concentration and the vulnerable individuals.
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Do not drink the untreated well water. At 14 mg/L nitrate-nitrogen, the water is 40% above the 10 mg/L guideline, so the family should switch to bottled or municipal water immediately, especially for the pregnant adult and the 2-year-old.
Why these individuals first:
- Infants under about six months are at risk of methemoglobinemia because their gut flora and hemoglobin are more susceptible to nitrite conversion; the 2-year-old is past the highest-risk window but still small and should not be exposed.
- Pregnancy is a period of high water intake and fetal sensitivity, so the pregnant adult should also avoid the water.
Practical measures:
- Use bottled water for drinking, cooking, and infant formula preparation.
- Do not assume boiling helps - boiling concentrates nitrate, so it makes the problem worse.
- Arrange a confirmatory resample at an accredited lab, since a single sample can be affected by handling or seasonal flushing.
- Ask a qualified contractor about a reverse-osmosis or ion-exchange treatment unit, or about extending and sealing the casing deeper into a confined aquifer.
- Notify the local health authority and consider testing the neighbour's well again after spring melt.
These steps protect health now while the long-term fix - source control, well reconstruction, or treatment - is planned.